1. Field of the Invention
This invention relates to fluid measuring instruments using a vibratable-wire as the basic force-sensing element. More particularly, this invention relates to compensating the vibration frequency of the wire for changes in ambient temperature.
2. Description of the Prior Art
The use of a vibratable-wire as the basic sensing element for pressure-responsive instruments has been often proposed by the prior art. Such proposals were based on the understanding that the frequency of wire vibration was closely related to the tension in the wire and, the recognition that the wire tension could in turn be controlled by a differential pressure to be measured. Thus, the frequency of wire vibration could be developed as a measurement signal responsive to differential pressure.
One such instrument is set forth in copending application Ser. No. 834,481, filed by E. O. Olsen et al, on Sept. 19, 1977. Here, the vibratable-wire is contained in a liquid-filled tube. While this instrument is suited for most industrial applications, it is less effective in applications involving extreme changes in ambient temperature. This is due to the effects of variations in the viscosity of the fill-liquid with changes in ambient temperature on the frequency of vibration, and thus on the accuracy of the pressure measurement.
Typically, viscosity increases with decreasing temperatures. Increases in viscosity result in a proportional reduction of the vibration frequency. Essentially, this is due to the fact that as the temperature decreases the liquid molecules become more viscous. The more viscous the liquid molecules, the more they adhere to the vibratable-wire thereby effectively increasing the mass of the wire. Since the vibrating frequency is inversely related to the mass of the wire, an increase in viscosity, other things being equal, decreases the frequency. The effect is greatly exhibited at the low end of the industrial temperature range (i.e., -40.degree. C. to +25.degree. C.). For example, liquid viscosity may change by 200% to 300% when the temperature drops from room temperature to -40.degree. C.
To reduce these temperature-dependent effects, the above mentioned application teaches the use of a fill-liquid characterized by (1) having a low room temperature viscosity and (2) a low viscosity temperature coefficient. While these compensating techniques are effective for most industrial applications, there remains some residual error due to extreme changes in ambient temperature.